It is known to control the arc in automatic production welding machines by means of a magnetic field. This can be done in fusion, butt, roll-seam, TIG, MIG, MAG, and friction-end welding. The procedure is frequently applied to the welding of tubes or workpieces with tubular projections, although it is not limited to rotation-symmetrical elements.
Two main methods are used for welding with a magnetically guided welding arc. The arc can either be drawn between the two workpieces being welded or can be drawn between a nonconsumed auxiliary electrode and the workpieces to be joined. In either situation the techniques of magnetically guiding the arc can be combined with those of the known resistance, arc, and friction welding advantageously.
The wandering or rotation of the arc is caused by well-known magnetic principles. Every current flow is accompanied by a magnetic field, this field forming concentric circles about a circular conductor, which an electric arc of a welding process can be considered to be. If this arc, which is generating its own electric field, passes through another stationary magnetic field, the magnetic lines of force will align themselves. The magnetic field lines tend to shorten themselves, so that the arc will become stationary within the additional fixed magnetic field. The force of this field can be determined by means of the well-known left-hand rule.
Particular difficulties are encountered in the welding of relativley thick workpieces, where the shape of the edge to be welded can vary considerably. To date it has been impossible to heat the entire end surface to be welded uniformly within a limited time period. For maximum efficiency the welding time of a magnetically guided arc, that is the actual burn time for the arc, should be held at a minimum. Thus once the arc has been struck it should move with a uniform speed about the surface to be welded so that the entire surface is uniformly treated and uniform results will be obtained from weld to weld.
German Pat. No. 129,179 discloses an arrangement where at the end of a so-called prewarming stage the current density is increased considerably, so as to constitute a sort of current shock. This current shock liquefies the previously uniformly heated welding surfaces and is self-limited by the subsequent upsetting operation. The problem with this system lies in striking of the arc, as with limited current density it is difficult to determine the exact instant at which the arc will be drawn, so that the extent to which the workpiece will be warmed before the above-mentioned current shock will vary from workpiece to workpiece, so that the welds themselves will also vary. Obviously this can be overcome by increasing the welding time per item, however this increase will decrease production efficiency which makes it undesirable.
U.S. Pat. No. 3,484,578 discloses a method wherein at the beginning of the welding operation the welding current is maintained relatively low while a relatively strong magnetic field is applied. In this manner striking of the arc is relatively easy. Nonetheless with this system, as with the system of the above-discussed German patent, it is necessary to strike the arc to either space the two elements between which the arc is to be drawn relatively closely, or to employ a relatively high tension. As a result it is difficult to form a uniform weld. Furthermore at the end of the welding operation the current increase has to be considerable, while regulating the speed of travel of the arc becomes quite difficult.
In practice it has been found necessary to space the parts being welded together by a distance between 2 mm and 3 mm. When, however, the welding current is limited it becomes difficult to strike an arc under these circumstances. Even when a relatively high starting voltage is employed striking the arc is problematic.